Photophysical behavior of open-shell first-row transition-metal octabutoxynaphthalocyanines: CoNc(OBu)8 and CuNc(OBu)8 as case studies

Article abstract of DOI:10.1021/ic7023204

Ultrafast photodynamics and density functional theory/time-dependent density functional theory (DFT/TDDFT) results for complexes of Co and Cu with 5,9,14,18,23,27,32,36-octabutoxynaphthalocyanine [CoNc(OBu)₈ and CuNc(OBu)₈] are reported. As a basis for this work, details concerning the syntheses of these complexes and the corresponding Zn complex (used as a reference) are given. Transient absorption spectrometry with femtosecond time resolution combined with a detailed DFT/TDDFT analysis has been employed to construct a complete picture of the excited-state dynamics after Q-band excitation of the Co and Cu complexes and to gain an understanding of the relationship between the nature of the metal center and the excited-state lifetime. The Co complex was shown to return to its ground state via two competing channels: a ²T₁(π, π*) state that decayed with a lifetime of 1 ps and a low-lying ²(d, d) state that repopulated the ground-state surface with a lifetime of 15 ps. CuNc(OBu) ₈ showed ground-state repopulation from the ²T ₁(π, π*) state via a lower-lying ligand-to-metal charge-transfer (LMCT) state that was completed within a few nanoseconds. The photophysical behavior of the cobalt and copper complexes was compared to that previously reported for the nickel analog in an effort to highlight the effect of the central metal on the nature and rates of the deactivation pathways. The results described in this work provide basic knowledge that is relevant to the use of these compounds as photothermal sensitizers in cancer therapy.

Full text of DOI:10.1021/ic7023204

Inorg. Chem. 2008, 47, 4275-4289
Photophysical Behavior of Open-Shell First-Row Transition-Metal
Octabutoxynaphthalocyanines: CoNc(OBu)8 and CuNc(OBu)8 as Case
Studies
Alexandra V. Soldatova,†,| Junhwan Kim,‡ Angela Rosa,*,§ Giampaolo Ricciardi,*,§
Malcolm E. Kenney,*,‡ and Michael A. J. Rodgers*,†
Center for Photochemical Sciences and Department of Chemistry, Bowling Green State UniVersity,
Bowling Green, Ohio 43403, Department of Chemistry, Case Western ReserVe UniVersity,
CleVeland, Ohio 44106, and Dipartimento di Chimica, UniVersità della Basilicata,
Via N. Sauro 85, 85100 Potenza, Italy
Received November 26, 2007
Ultrafast photodynamics and density functional theory/time-dependent density functional theory (DFT/TDDFT) results
for complexes of Co and Cu with 5,9,14,18,23,27,32,36-octabutoxynaphthalocyanine [CoNc(OBu)8 and CuNc(OBu)8]
are reported. As a basis for this work, details concerning the syntheses of these complexes and the corresponding
Zn complex (used as a reference) are given. Transient absorption spectrometry with femtosecond time resolution
combined with a detailed DFT/TDDFT analysis has been employed to construct a complete picture of the excited-
state dynamics after Q-band excitation of the Co and Cu complexes and to gain an understanding of the relationship
between the nature of the metal center and the excited-state lifetime. The Co complex was shown to return to its
2
ground state via two competing channels: a T1(π, π*) state that decayed with a lifetime of 1 ps and a low-lying
2(d, d) state that repopulated the ground-state surface with a lifetime of 15 ps. CuNc(OBu)8 showed ground-state
2
repopulation from the T1(π, π*) state via a lower-lying ligand-to-metal charge-transfer (LMCT) state that was
completed within a few nanoseconds. The photophysical behavior of the cobalt and copper complexes was compared
to that previously reported for the nickel analog in an effort to highlight the effect of the central metal on the nature
and rates of the deactivation pathways. The results described in this work provide basic knowledge that is relevant
to the use of these compounds as photothermal sensitizers in cancer therapy.
Introduction
have been shown to have great potential, and some of them
are already in phase-I clinical trials.2–6 Recent results on one
potential phototherapeutic compound from the Pc family,
Ni(II) octabutoxynaphthalocyanine [NiNc(OBu)8], indicate
that after this compound is taken up by cancer cells, it can
cause destruction of these cells through photothermal events.7–9
Since their discovery in the early 20th century, metal-
lophthalocyanines (MPc’s) have found diverse applications1
as a result of the inherent chemical flexibility of the
metallomacrocycle, which allows modulation of their phys-
icochemical properties through ligand and metal modifica-
tion. One particular application of phthalocyanines to date
lies in the field of photomedicine as it relates to cancer
treatment. In photodynamic therapy, for example, MPc’s
(2) Szacilowski, K.; Macyk, W.; Drzawiecka-Matuszek, A.; Brindell, M.;
Stochel, G. Chem. ReV. 2005, 105, 2647.
(3) Ali, H.; Lier, J. E. Chem. ReV. 1999, 99, 2379.
(4) Lukyanets, E. A. J. Porphyrins Phthalocyanines 1999, 3, 424.
(5) Rosenthal, I. J. Photochem. Photobiol. 1991, 53, 859.
(6) Nyman, E. S.; Hynninen, P. H. J. Photochem. Photobiol., B 2004,
73, 1.
* To whom correspondence should be addressed. E-mail: rosa@unibas.it
(A.R.), rg010sci@unibas.it (G.R.), malcolm.kenney@case.edu (M.E.K.),
rodgers@bgnet.bgsu.edu (M.A.J.R.).
†
Bowling Green State University.
Case Western Reserve University.
Università della Basilicata.
‡
(7) Busetti, A.; Soncin, M.; Reddi, E.; Rodgers, M. A. J.; Kenney, M. E.;
Jori, G. J. Photochem. Photobiol. B 1999, 53, 103.
(8) Camerin, M.; Rello, S.; Villaneuva, A.; Ping, X.; Kenney, M. E.;
Rodgers, M. A. J.; Jori, G. Eur. J. Cancer 2005, 41, 1203.
(9) Camerin, M.; Rodgers, M. A. J.; Kenney, M. E.; Jori, G. Photochem.
Photobiol. Sci. 2005, 4, 251.
§
| Present address: Department of Chemistry, University of Washington,
Seattle, WA 98195.
(1) Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever,
A. B. P., Eds.; VCH Publishers: New York, 1990–1996; Vols. 1–4.
10.1021/ic7023204 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 10, 2008 4275
Published on Web 04/12/2008

Soldatova et al.
Scheme 1. Synthetic Route for Metal Naphthalocyanines
It is generally understood that for a compound to be an
effective photothermal agent, it not only should possess high
photostability and strong absorption in the phototherapeutic
spectral window but also should be able to very rapidly return
to its ground state after photoexcitation, thus generating a
vibrationally hot ground-state molecule. NiNc(OBu)8 is a
typical example of a molecule that undergoes ultrafast
radiationless deactivation of its excited states after photon
absorption. We recently investigated the excited-state deac-
tivation mechanism of NiNc(OBu)8 using ultrafast measure-
ments and density functional theory/time-dependent density
functional theory (DFT/TDDFT) calculations.10 Transient
absorption results showed that after photoexcitation of
NiNc(OBu)8 in toluene, ground-state repopulation occurred
with a lifetime of 500 ps. Integration of the TDDFT and
experimental results indicated that the photoproduced S1(π,
exchange-correlation (XC) potential11,12 and recently imple-
mented in the Amsterdam Density Functional (ADF) code.13
Experimental Section
A. Materials for Photochemical Studies. The solvents toluene
(99.5+%, Aldrich spectrophotometric grade), benzene (99%, Al-
drich HPLC grade), dichloromethane (99%, Aldrich HPLC grade),
and benzonitrile (99%, Aldrich HPLC grade) were used as received.
B. Synthesis of Naphthalocyanines. Metal naphthalocyanines
were prepared by metal insertion reactions, as shown in Scheme 1.
Full descriptions of the syntheses are given in ref 14.
π*) excited state undergoes fast deactivation through a
3
2
2
2
cascade of events involving the lower-lying (dz , dx –y ), T1(π,
3
2
2
π*), and LMCT(π, dx –y ) excited states. The close proximity
3
2
2
2
of the (dz , dx –y ) and S1(π, π*) states facilitates the process
of intersystem crossing to the triplet manifold, with the first
observed transient being a vibrationally excited T1(π, π*)
1,4-Dihydroxy-2,3-naphthalenedicarbonitrile (1).15 1 is a
yellow-white solid that is soluble in dimethylformamide, slightly
soluble in CH2Cl2, and insoluble in toluene and hexanes. Yield:
4.1 g, 85%. NMR [50:1 CDCl3-(CD3)2SO]: δ 8.07 (m, 2H, 5,8-
ArH), 7.44 (m, 2H, 6,7-ArH).
state. After vibrational relaxation, the T1(π, π*) state converts
3
2
2
rapidly and reversibly into the LMCT(π, dx –y ) state. The
equilibrium state so generated decays to the ground state with
a lifetime of ∼500 ps.
1,4-Dibutoxy-2,3-naphthalenedicarbonitrile (2).16,17 2 is a
white solid that is soluble in CH2Cl2, dimethylformamide, toluene,
and hexanes. Yield: 5.0 g, 80%. NMR (CDCl3): δ 8.25 (m, 2H,
5,8-ArH), 7.78 (m, 2H, 6,7-ArH), 4.45 (t, 4H, OCH2), 1.95 (m,
4H, OCH2CH2), 1.62 (m, 4H, OC2H4CH2), 1.04 (t, 6H, OC3H6CH3).
H2Nc(OBu)8 (3).16,17 Under Ar, a mixture of nitrile 2 (4.7 g)
and a solution of NaOCH3 in CH3OH (0.50 M, 18 mL), which had
been diluted with 1-butanol (30 mL) and then distilled until free
of CH3OH, was slowly distilled (5 h, 25 mL of distillate), diluted
with CH3OH (30 mL), and filtered. The brown solid was washed
in CH3OH, chromatographed using Al2O3-III and toluene, vacuum-
dried at room temperature, and weighed. 3 is soluble in CH2Cl2,
dimethylformamide, and toluene and slightly soluble in hexanes.
Yield: 3.1 g, 65%. UV–vis (toluene): λmax (log ꢀ) 864 nm (5.5).
NMR (CDCl3): δ 8.94 (m, 8H, 1,4-NcH), 7.86 (m, 8H, 2,3-NcH),
5.13 (t, 16H, OCH2), 2.21 (m, 16H, OCH2CH2), 1.64 (m, 16H,
OC2H4CH2), 1.01 (t, 24H, OC3H6CH3). HRMS-ESI-TOF for
[M]+ with M as C80H90N8O8: calcd, m/z 1290.6881; found, m/z
1290.6825.
The deactivation mechanism proposed for NiNc(OBu)8
indicated that the relative positions of the metal-centered
(MC) and ligand-to-metal charge-transfer (LMCT) excited
states with respect to either the photogenerated S1(π, π*)
state or the T1(π, π*) state are crucial for fast dissipation of
the photon energy by thermal radiationless events. The
number and energies of MC, LMCT, and MLCT excited
states lying below the S1(π, π*) state depend on the central
metal. Therefore, the photodeactivation mechanism and
hence the efficiency of MNc(OBu)8 complexes as photo-
thermal sensitizers is expected to vary upon replacement of
nickel by other first-row transition metals. In the present
work, the deactivation mechanisms of the photoexcited
cobalt(II) and copper(II) octabutoxynaphthalocyanine deriva-
tives, CoNc(OBu)8 and CuNc(OBu)8, both of which have a
doublet ground state, are reported for the first time and
compared with the mechanism recently reported for the Ni(II)
complex.
The excited-state spectral and dynamic behaviors of the
title complexes have been investigated by ultrafast transient
absorption spectroscopy and interpreted with the aid of
TDDFT calculations of the doublet and quartet excited states
lying below the photogenerated 2S1(π, π*) state. The
properties of the quartet excited states, which involve spin-
flip (SF) transitions, have been studied using a TDDFT
formalism based on the noncollinear representation of the
CoNc(OBu)8 (4).18 Under Ar, a mixture of naphthalocyanine 3
(42 mg), Co(CH3CO2)2 ·4H2O (12 mg), 1,8-diazabicyclo[5.4.0]undec-
7-ene (0.02 mL), and 1-butanol (2 mL) was refluxed for 2 h, diluted
with a solution of CH3OH and H2O (1:1, 6 mL), and filtered. The
green solid was chromatographed using Al2O3-III and toluene,
rechromatographedusingAl2O3-IIIand10:1hexanes-tetrahydrofuran,
(13) ADF2006.01; Scientific Computing & Modelling NV: Amsterdam,
2006; available from http://www.scm.com.
(14) Kim, J. Ph.D. Thesis, Case Western Reserve University, 2006.
(15) Reynolds, G. A.; Vanallan, J. A. J. Org. Chem. 1964, 29, 3591.
(16) Rihter, B. D.; Kenney, M. E.; Ford, W. E.; Rodgers, M. A. J. J. Am.
Chem. Soc. 1993, 115, 8146.
(10) Soldatova, A. V.; Kim, K.; Peng, X.; Rosa, A.; Ricciardi, G.; Kenney,
M. E.; Rodgers, M. A. J. Inorg. Chem. 2007, 46, 2080.
(17) Cook, M. J.; Dunn, A. J.; Howe, S. D.; Thomson, A. J.; Harrison,
K. J. J. Chem. Soc., Perkin Trans. 1988, 1, 2453.
(18) Gao, D.; Zhao, H.; Huo, L.; Zhao, J. G.; Wu, Y. Q.; Xi, S. Q. Sens.
Actuators, B 2004, 97, 319.
(11) Wang, F.; Ziegler, T. J. Chem. Phys. 2004, 121, 12191.
(12) Wang, F.; Ziegler, T. J. Chem. Phys. 2005, 122, 074109.
4276 Inorganic Chemistry, Vol. 47, No. 10, 2008

Photophysical BehaWior of CoNc(OBu)8 and CuNc(OBu)8
vacuum-dried at room temperature, and weighed. 4 is soluble in
CH2Cl2, dimethylformamide, and toluene and slightly soluble in
hexanes. Yield: 35 mg, 78%. UV–vis (toluene): λmax (log ꢀ) 830
nm (4.6). HRMS-ESI-TOF for [M]+ with M as C80H88N8O859Co:
calcd, m/z 1347.6057; found, m/z 1347.6029.
source, and near-IR signals were monitored at the right-angle
geometry using a Peltier-cooled InGaAs detector with lock-in
amplification. Luminescence spectra were acquired at room tem-
perature, and all solutions were saturated with Ar prior to
luminescence experiments. Appropriate filters were used to prevent
scattered light from entering the detection system. All of the spectra
were corrected for the sensitivity of the detector.
NiNc(OBu)8 (5). See ref 10.
CuNc(OBu)8 (6).19 Under Ar, a mixture of naphthalocyanine 3
(110 mg), Cu(CH3CO2)2 ·H2O (23 mg), 1,8-diazabicyclo[5.4.0]undec-
7-ene (0.05 mL), and 1-butanol (5 mL) was refluxed for 20 h,
diluted with a solution of CH3OH and H2O (1:1, 15 mL), and
filtered. The brown solid was chromatographed using Al2O3-V and
toluene, vacuum-dried at room temperature, and weighed. 6 is
soluble in CH2Cl2, dimethylformamide, and toluene and slightly
soluble in hexanes. Yield: 72 mg, 62%. UV–vis (toluene): λmax (log
ꢀ) 850 nm (5.4). HRMS-ESI-TOF for [M]+ with M as
C80H88N8O863Cu: calcd, m/z 1351.6022; found, m/z 1351.5986.
ZnNc(OBu)8 (7).20 Under Ar, a mixture of naphthalocyanine 3
(100 mg), Zn(CH3CO)2 ·2H2O (23 mg), 1,8-diazabicyclo[5.4.0]undec-
7-ene (0.05 mL), and 1-butanol (5 mL) was refluxed for 4 h, diluted
with a solution of CH3OH and H2O (1:1, 15 mL), and filtered. The
brown solid was chromatographed using Al2O3-III and 1:1
tetrahydrofuran-hexanes, rechromatographed using Al2O3-III and
1:5 tetrahydrofuran-hexanes, vacuum-dried at room temperature,
and weighed. 7 is soluble in CH2Cl2, dimethylformamide, and
toluene and slightly soluble in hexanes. Yield: 69 mg, 66%. UV–vis
(toluene): λmax (log ꢀ) 846 nm (4.9). NMR (CDCl3): δ 9.00 (m,
8H, 1,4-NcH), 7.88 (m, 8H, 2,3-NcH), 5.25 (t, 16H, OCH2), 2.25
(m, 16H, OCH2CH2), 1.68 (m, 16H, OC2H4CH2), 1.05 (t, 24H,
OC3H6CH3). HRMS-ESI-TOF for [M]+ with M as C80H88-
N8O864Zn: calcd, m/z 1352.6017; found, m/z 1352.6024.
C. UV–Visible and Near-IR Absorption Spectra. Ground-state
electronic absorption spectra in the visible region were recorded
using a Varian Cary 50 Bio UV–vis single-beam spectrophotometer
(Varian Corp.). Near-IR spectra were recorded using a PerkinElmer
Lambda 19 UV–vis-NIR absorption spectrometer in the laboratory
of the Chisholm research group at The Ohio State University. In
each measurement, the sample was referenced to a matched cuvette
containing pure solvent. To observe optically forbidden transitions
in the near-IR region, samples with concentrations of ∼150 µM
were employed. No aggregation phenomena were observed under
such conditions.
The fluorescence quantum efficiency of CuNc(OBu)8 was
measured using ZnNc(OBu)8 in toluene as a standard. The reference
and the sample were excited at 458 nm, where they had the same
absorbance (A ) 0.02). The areas under the fluorescence spectra
(G) were measured, and fluorescence quantum efficiencies (qM)
were calculated according to the equation
A
Gsample
Asample
ZnNc(OBu)8
qsMample
)
qZMnNc(OBu)
(2)
8
(
)
(
)
G
ZnNc(OBu)8
The value for the absolute fluorescence quantum efficiency for the
Zn complex was estimated using the Strickler–Berg equation:22
nf3
1
τrad
ꢀ dν
ν
) krad ) 2.88 × 10-9
ν -3
(3)
-1
〈
〉
∫
f
( )
na
The calculated radiative lifetime was found to be ∼8.45 ns, in good
agreement with the radiative lifetime of 8.8 ns for unsubstituted
ZnNc measured in DMF.23 The singlet-state lifetime of 0.6 ns
measured using transient absorption spectrometry was employed
to calculate a value of qM ) 0.07 for the absolute quantum efficiency
of the Zn complex.
For the 2,4T(π, π*) near-IR emission measurements, Xe arc
excitation (830 nm) was used and the concentration of naphthalo-
cyanines was ∼6 µM, providing an optical absorbance of ∼1.2 in
the Q-band maximum.
E. Ultrafast Pump–Probe Measurements. Details of the
pump–probe instrument for ultrafast transient absorption measure-
ments at the Ohio Laboratory for Kinetic Spectrometry at Bowling
Green State University have been described previously.24 Recent
improvements to enhance signal-to-noise characteristics have been
published elsewhere.25 For the present measurements, the main part
(95%) of the fundamental output beam (95 fs, 1 kHz, 800 nm)
from an amplified, mode-locked Ti:sapphire laser (Hurricane,
Spectra-Physics) was used to excite the sample or to pump an
optical parametric amplifier (OPA 800, Spectra Physics). An OPA
was used to generate excitation pulses in the range 800–900 nm
(the second harmonic of the idler). The pump intensity was
attenuated to give an energy of 1–3 µJ/pulse at the sample. The
transient absorption signal was monitored with the white-light
continuum probe in the 460–780 nm spectral range or with the
extended to the near-IR continuum in the region of 800–1100 nm.
The instrument response time of the ultrafast spectrometer was
∼200 fs. The concentrations of the naphthalocyanines were typically
∼20–30 µM in the 2 mm path length cuvette. Absorption spectra
of the solutions were measured before and after the experiment to
check for possible sample degradation.
Oscillator strengths, f, of the Q and near-IR absorption bands of
the two complexes were estimated by fitting the experimental
spectral bands with a single Gaussian function and using the
expression
2
f ) (3.32 × 10-9
)
e-(∆ν ⁄ θ) dν ) (3.32 × 10-9) πꢀmax
θ
(1)
√
∫
where θ is the half-width at ꢀ ) ꢀmax/e. The width parameter θ is
related to the full width (h) at half-maximum (ꢀmax/2) by the
expression h ) 2(ln 2)1/2θ.
The oscillator strengths obtained from eq 1 neglect the solvent
refractive index correction. For common solvents, the inclusion of
the refractive index correction may result in changes on the order
of 10–20%.21
D. Luminescence Spectra. Steady-state near-IR luminescence
spectra were obtained using a PTI Instruments QM-4/2006-SE
spectrofluorimeter. Samples in a 10 mm × 10 mm path length
quartz cuvette were excited using a 75 W Xe arc lamp excitation
F. Quantum Chemical Calculations. All of the calculations
were performed with the ADF program package.13,26 Geometry
(22) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.
(23) Kumar, G. A.; Santhosh, C. Mater. Lett. 2003, 57, 2315.
(24) Nikolaitchik, A. V.; Korth, O.; Rodgers, M. A. J. J. Phys. Chem. A
1999, 103, 7587.
(25) Pelliccioli, A. P.; Henbest, K.; Kwag, G.; Carvagno, T. R.; Kenney,
M. E.; Rodgers, M. A. J. J. Phys. Chem. A 2001, 105, 1757.
(26) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra,
C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput.
Chem. 2001, 22, 931.
(19) NiNc(OBu)8 and CuNc(OBu)8 are available commercially but were
prepared in this study to assure the purity desired.
(20) Yang, S.; Liu, E.; Chen, N.; Huang, J.; Pu, P.; Duan, W.; Wu, Y. Q.;
Zuo, X.; Mao, G. Chin. J. Inorg. Chem. 1997, 13, 83.
(21) McHugh, A. J.; Gouterman, M. Theor. Chim. Acta 1969, 13, 249.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4277

Soldatova et al.
optimizations and ground-state electronic structure calculations were
performed using the Vosko-Wilk-Nusair (VWN) local density
approximation (LDA) functional27 plus the Becke88-Perdew86
(BP) generalized gradient approximation (GGA).28,29 The spin-
unrestricted DFT formalism was employed to examine the open-
shell Co and Cu complexes. The model compounds MNc(OMe)8
(M ) Co, Cu; Me ) methyl), in which the butyl substituents of
the title compounds were replaced by methyl groups, were chosen
in order to facilitate the calculations. Structure optimizations were
performed using no constraints, starting from nonsymmetric
structures in which methyl substituents residing on the same
naphthoindole subunit had the same orientation as in NiNc(OMe)8.10
The structures of CoNc(OMe)8 and CuNc(OMe)8 were optimized
1
by assuming an S ) /2 spin state.
Excitation energies were calculated using ordinary (spin-
unrestricted) TDDFT11 and spin-unrestricted SF-TDDFT based on
a noncollinear representation of the XC potential.11,12 In ADF, the
SF-TDDFT approach is implemented within the Tamm-Dancoff
approximation (TDA).30 In the TDDFT calculations, the same GGA
as in the DFT calculations was used for the XC potential, which
appears in the zeroth-order Kohn–Sham (KS) equations. Recent
studies of the ground and excited states of cobalt31–35 and nickel
tetrapyrroles10,36–38 have demonstrated that nonhybrid functionals,
including the BP functional used in this study, provide results that
are very consistent with experiment. For the so-called XC kernel,
the first functional derivative of the time-dependent XC potential
with respect to the time-dependent density F(r′, t′), the adiabatic
local density approximation (ALDA) was employed.
Figure 1. Normalized ground-state absorption spectra of CoNc(OBu)8,
NiNc(OBu)8, and CuNc(OBu)8 in toluene solution in the Q-band region.
using the structures optimized in the gas phase. In the ADF
implementation, COSMO surface charges are not explicitly con-
sidered in the TDDFT procedure other than through their influence
on the ground-state density.41 This effectively amounts to using
the ground-state surface charges for both the ground and excited
states as a first approximation to simulate the effects of the solvent
on the excitation energies. The high-frequency component of the
solvent electric susceptibility, which accounts for changes in the
electronic structure of the solvent upon electronic excitations within
the solute, was not considered.
The DFT and TDDFT calculations employed the all-electron
ADF TZ2P basis set, which is an uncontracted triple-ꢀ STO basis
set having one 3d and one 4f polarization function for each C, N,
and O atom, one 2p and one 3d polarization function for each H
atom, and a triple-ꢀ 3d, 4s basis with one 4p and one 4f function
for Co and Cu.
Vertical absorption energies, Eva, were evaluated at the ground-
state optimized geometry. For selected excited states, adiabatic
energies, Eadia, were also computed according to the expression
Eadia ) Eve + ∆E
(4)
where Eve is the vertical emission energy, which is calculated at
the TDDFT level using the relaxed excited-state geometry. The
∆E term accounts for the change in energy of the ground state
upon deformation to the relaxed geometry of the excited state (for
a schematic definition of the calculated energies, see Figure 10 in
ref 38).
The optimized ground- and excited-state structures were verified
to represent local minima by calculation of harmonic frequencies.
Spin contamination, monitored by the expectation value of S2, was
found to be negligible in all cases.
Results and Discussion
A. Ground-State Absorption Spectra. The normalized
ground-state absorption spectra of the cobalt(II) and cop-
per(II) octabutoxynaphthalocyanine complexes in toluene in
the energy window of the Q band are displayed in Figure 1.
For comparison purposes, the optical absorption spectrum
of the nickel(II) analog is also given. The complexes
exhibited a very intense Q(0, 0) band in the 830–850 nm
region with a weak vibrational progression toward the blue.
The extinction coefficients of the Q(0, 0) band in toluene
were 1.68 × 105 M-1 cm-1 (832 nm), 2.25 × 105 M-1 cm-1
(845 nm), and 2.31 × 105 M-1 cm-1 (851 nm) for the Co,
Ni, and Cu complexes, respectively. Notably, the Q-band
system experienced a clear red shift across the MNc(OBu)8
(M ) Co, Ni, Cu) series.
In addition to the strongly allowed transition to the Q state,
CoNc(Bu)8 and CuNc(OBu)8 spectra exhibited weak bands
in the near-IR region (Figure 2). The copper complex showed
two distinct absorptions, one at 1155 nm (ꢀ ≈ 250 M-1 cm-1)
and the other at 1440 nm (ꢀ ≈ 150 M-1 cm-1). On the basis
of the emission data and TDDFT calculations (described
below), these bands were assigned to the “tripdoublet”, 2T1(π,
Effects of the solvent on the excitation energies were modeled
with the conductor-like continuum solvent model (COSMO),39–41
(27) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(28) Becke, A. Phys. ReV. A 1988, 38, 3098.
(29) Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
(30) Hirata, S.; Head-Gordon, M. Chem. Phys. Lett. 1999, 314, 291.
(31) Andruniow, T.; Kozlowski, P. M.; Zgierski, M. Z. J. Chem. Phys.
2001, 115, 7522.
(32) Andruniow, T.; Zgierski, M. Z.; Kozlowski, P. M. J. Phys. Chem. A
2002, 106, 1365.
(33) Kozlowski, P. M.; Andruniow, T.; Jarzecki, A. A.; Zgierski, M. Z.;
Spiro, T. G. Inorg. Chem. 2006, 45, 5585.
(34) Kuta, J.; Patchkovskii, S.; Zgierski, M. Z.; Kozlowski, P. M. J. Comput.
Chem. 2006, 27, 1429.
(35) Jaworska, M.; Lodowski, P.; Andruniow, T.; Kozlowski, P. M. J. Phys.
Chem. B 2007, 111, 2419.
(36) Patchkovskii, S.; Kozlowski, P. M.; Zgierski, M. Z. J. Chem. Phys.
2004, 121, 1317.
(37) Rosa, A.; Ricciardi, G.; Baerends, E. J.; Zimin, M.; Rodgers, M. A. J.;
Matsumoto, S.; Ono, N. Inorg. Chem. 2005, 44, 6609.
(38) Gunaratne, T. C.; Gusev, A. V.; Peng, X.; Rosa, A.; Ricciardi, G.;
Baerends, E. J.; Rizzoli, C.; Kenney, M. E.; Rodgers, M. A. J. J. Phys.
Chem. A 2005, 109, 2078.
(40) Klamt, A.; Jonas, V. J. Chem. Phys. 1996, 105, 9972.
(41) Pye, C. C.; Ziegler, T. Theor. Chem. Acc. 1999, 101, 396.
(39) Klamt, A.; Schürmann, G. J. Chem. Soc., Perkin Trans. 1993, 2, 799.
4278 Inorganic Chemistry, Vol. 47, No. 10, 2008

Photophysical BehaWior of CoNc(OBu)8 and CuNc(OBu)8
equation (see Luminescence Spectra in the Experimental
Section). The CuNc(OBu)8 fluorescence quantum efficiency
obtained in this way was ∼3 × 10-4, implying that effective
nonradiative channels exist for deactivation of the Q state.
Upon excitation at 830 nm, a 6 µM solution of the copper
complex in Ar-saturated benzene showed a clear lumines-
cence band at 1457 nm, most likely due to emission from
4
the T(π, π*) state. In addition, a poorly resolved shoulder
in the fluorescence tail could be seen at around 1175 nm. It
was tempting to assign this feature to emission from the
2T1(π, π*) state, although, in view of its weakness, the
emission might arise from the presence of emissive impurities.
Under the same experimental conditions, the cobalt
2
2
complex did not show the S1 f S0 fluorescence signal.
However, a weak emission band at 1092 nm, presumably
from the 2T1(π, π*) state, could be resolved upon excitation
of the complex in Ar-saturated benzene at 830 nm (not
shown); in the near-IR absorption spectrum of CoNc(OBu)8,
this band was hidden in the Q-band absorption tail.
C. Transient Absorption Experiments. Femtosecond
transient absorption spectrometry with excitation wavelengths
within the Q bands (800–845 nm) was employed to in-
vestigate the deactivation of the excited states of the Co(II)
and Cu(II) naphthalocyanine complexes. The transient ab-
sorption species formed were probed in the visible and near-
IR regions.
CuNc(OBu)8. Figure 4 shows transient absorption spectra
for CuNc(OBu)8 in toluene (35 µM) at different time delays
following excitation at 800 nm and probed in the visible
(Figure 4a) and near-IR (Figure 4b) regions of the spectrum.
The first observed transient in the visible region (3 ps)
displayed features characteristic of excited states local-
ized on phthalocyanine and naphthalocyanine macrocy-
cles,10,16,24,38,46–50 exhibiting a broad positive absorption
band with a maximum at 635 nm and two negative bands,
at 480 and 755 nm, arising from ground-state depopulation.
During the first picosecond or so, the positive absorption
signal at ∼650 nm was seen to grow (see Figure 4a, inset).
As the time delay between pump and probe was further
increased, the intensity of the transient absorption signal
decreased but no changes in its spectral shape were detected.
Two isosbestic points connecting the positive and negative
features, one at 510 nm and the other at 740 nm, were
maintained during the decay. In the near-IR region, the only
observed spectral feature was bleaching in the Q-band region
caused by ground-state depopulation; no spectral shift was
detected during the time window of the experiment.
Time profiles at 680 and 850 nm are shown in panels a
and b of Figure 5, respectively. The profiles are truncated
by the limit of the experimental time window. They were
Figure 2. Near-IR ground-state absorption spectra of ∼150 µM CoNc(OBu)8
and CuNc(OBu)8 in CH2Cl2.
π*), and quartet, 4T1(π, π*), excited states derived from the
π f π* transition between the highest-occupied and lowest-
unoccupied Gouterman molecular orbitals (G-HOMO and
G-LUMO, respectively). This transition also generates the
“singdoublet”, 2S1(π, π*), excited state that accounts for the
Q(0, 0) band.42,43 As expected, the T1(π, π*) state, which
2
4
is weakly allowed, acquired more intensity than the T1(π,
π*) state. In the Co complex, however, only one weak
absorption at 1332 nm (ꢀ ≈ 210 M-1 cm-1) could be resolved
in the near-IR spectrum, although one or more absorptions
are likely to contribute to the broadening of the red tail of
the Q band (see Figure 2). TDDFT calculations suggested
that the absorption at 1332 nm be assigned to the 4T1(π, π*)
excited state. Weak near-IR absorption bands have been
reported for cobalt(II) and copper(II) phthalocyanines and
assigned, mostly on the basis of emission data, to tripmul-
tiplets.44,45 Table 1 summarizes the absorption and emission
properties of the complexes.
B. NIR Emission Spectra of Co(OBu)8 and Cu(OBu)8.
The title compounds were examined for emission properties
at room temperature. No fluorescence was observed for the
cobalt complex, whereas CuNc(OBu)8 was weakly fluores-
cent with an emission maximum at ∼878 nm (the fluores-
cence spectrum was obtained using a monochromator slit
width of 10 mm). The emission and absorption spectra of
the Cu complex are brought together in Figure 3. The
quantum efficiency for fluorescence of CuNc(OBu)8 was
estimated using the ZnNc(OBu)8 complex as a standard; an
approximate value for the absolute quantum efficiency of
the Zn complex was obtained using the Strickler–Berg
(42) The classification of the (π, π*) excited states in terms of “singdou-
blets” and “tripdoublets” was originally introduced by Gouterman and
co-workers (see ref 43). Interaction between the 1(π, π*) states of the
macrocycle and the unpaired d electron on the metal produces doublet
states (singdoublets), while interaction between the 3(π, π*) states of
the macrocycle and the unpaired d electron yields doublets (tripdou-
blets) and quartets.
(46) Nikolaitchik, A. V.; Rodgers, M. A. J. J. Phys. Chem. A 1999, 103,
7597.
(47) Aoudia, M.; Cheng, G.; Kennedy, V. O.; Kenney, M. E.; Rodgers,
M. A. J. J. Am. Chem. Soc. 1997, 119, 6029.
(43) Gouterman, M.; Mathies, R. A.; Smith, B. E.; Caughey, W. S. J. Chem.
Phys. 1970, 52, 3795.
(48) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kenney, M. E.; Rodgers,
M. A. J. J. Am. Chem. Soc. 1988, 110, 7626.
(44) Vincett, P. S.; Voigt, E. M.; Rieckhoff, K. E. J. Chem. Phys. 1971,
55, 4131.
(49) Rihter, B. D.; Kenney, M. E.; Ford, W. E.; Rodgers, M. A. J. J. Am.
Chem. Soc. 1990, 112, 8064.
(45) Lever, A. B. P.; Pickens, S. R.; Minor, P. C.; Licoccia, S.; Ramaswamy,
B. S.; Magnell, K. J. Am. Chem. Soc. 1981, 103, 6800.
(50) Fournier, M.; Pepin, C.; Houde, D.; Ouellet, R.; Van Lier, J. E.
Photochem. Photobiol. Sci. 2004, 3, 120.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4279

Soldatova et al.
Table 1. Absorption and Emission Data for CoNc(OBu)8 and CuNc(OBu)8
absorption
emission
λmax [nm] (φ [%])
λmax [nm] (ꢀ [M-1 cm-1])
CoNc(OBu)8
CuNc(OBu)8
832 (1.68 × 105)a
851 (2.31 × 105)a
n/a
1332 (210)b
1440 (150)b
-
1092d
-
1155 (250)b
878 (0.03)c
1172e
1460d
a Measured in toluene at room temperature. b Measured in CH2Cl2 at room temperature. c Measured in Ar-saturated benzene at room temperature with λexc
) 458 nm. d Measured in Ar-saturated benzene at room temperature with λexc ) 830 nm. e Seen as a shoulder in the red tail of the fluorescence emission.
fitted to a zero baseline using a single-exponential function,
providing a lifetime of ∼1.1 ns.
As time progressed, the featureless positive absorption
in the visible region decayed to a zero baseline, while
the negative bands became blue-shifted during the decay
period. Eventually, a spectral transient with alternating
negative and positive lobes at 735 and 765 nm, charac-
teristic of a (d, d) state, remained (Figure 7a, inset). In
Figure 7b, the blue shift of the transient spectrum is more
obvious. As can be seen, during the first 5 ps, both the
negative absorption extreme and the broad positive
absorption band at ∼900 nm shifted toward the blue and
narrowed significantly. During this time, the difference
spectrum evolved into one having a derivative-like shape,
and no isosbestic behavior was observed. At later times,
the derivative-shaped spectrum decayed uniformly to the
baseline, reflecting complete ground-state recovery. The
inset in Figure 7a shows an isosbestic point near 755 nm,
Figure 6 displays the rise time of the 630 nm transient
superimposed on the time profile of a solvent-derived Raman
signal that shows the instrument response profile. As can be
seen, the increase in the transient absorption at 630 nm occurs
more slowly than the ∼200 fs solvent response. Experimental
points were fitted starting from the time when the Raman
signal disappeared, leading to an estimate of 200 fs for the
rise time of the 630 nm absorption.
CoNc(OBu)8. Figure 7a shows overlaid transient absorp-
tion spectra in the visible region at a series of delay times
after excitation of CoNc(OBu)8 in toluene (23 µM) using
832 nm pulses. Formation of the positive absorption band
and the ground-state bleaching was complete within the
instrument response time (∼200 fs). The first observed
transient in the visible region (at 1.2 ps in Figure 7a) revealed
spectral features similar to those reported above for CuN-
c(OBu)8 and previously10 for NiNc(OBu)8, i.e., a broad
positive absorption centered at 650 nm and negative absorp-
tion bands at 470 and 745 nm arising from ground-state
bleaching.
Figure 7b shows a series of time-delayed spectra in the
near-IR region. In addition to the negative-absorption spectral
feature arising from bleaching of the Q band, the spectrum
of the first observed transient was also characterized by a
broad positive absorption to the red of the bleaching
maximum.
Figure 3. Ground-state absorption spectra (black lines) and static emission
spectra (red and pink lines) of CuNc(OBu)8. Ground-state absorption spectra
were measured in toluene (Q-band region) and CH2Cl2 (near-IR region).
Emission spectra were obtained in Ar-saturated benzene with excitation
wavelengths of 458 nm (fluorescence emission) and 830 nm (near-IR
emission). Concentrations of ∼0.5 µM [AQ(0,0) ) 0.12] and ∼6 µM were
used for the fluorescence and near-IR emission measurements, respectively.
The fluorescence spectrum was normalized to the Q-band absorption
maximum, and the near-IR emission spectrum was normalized to the
maximum of the lowest-energy absorption band (1440 nm).
Figure 4. Evolution of the transient absorption spectra of CuNc(OBu)8 in
toluene (∼35 µM) upon photoexcitation at 800 nm using a pump energy of
5 µJ/pulse. (a) Transient absorption spectra probed in the visible region.
Inset: transient absorption evolution showing growth of the positive feature
during the first picosecond of the experiment. (b) Transient absorption
spectra probed in the near-IR region at various delay times.
4280 Inorganic Chemistry, Vol. 47, No. 10, 2008

Photophysical BehaWior of CoNc(OBu)8 and CuNc(OBu)8
Figure 5. Kinetic profiles of the transient absorption signals at probe wavelengths of (a) 680 and (b) 850 nm for CuNc(OBu)8 in toluene after excitation
at 800 nm. The red lines are fits to the experimental data points and blue lines are residuals for the fits.
Figure 6. Kinetic profile of the transient absorption of CuNc(OBu)8 in
toluene solution at 630 nm. The blue dashed curve is the instrument response
to a solvent Raman signal (∼200 fs). Both traces are normalized to unity
to demonstrate the differences in response time.
indicating concomitant behavior of the decay of the
positive absorption and the ground-state repopulation.
The spectral evolution described above was accompanied
by complex temporal behavior having wavelength-dependent
lifetimes. Temporal decay profiles recorded at 650 and 900
nm are shown in panels a and b of Figure 8, respectively.
The positive absorption in the visible region (Figure 8a)
decayed to a zero baseline with single-exponential behavior
having a lifetime of 1.0 ps, which showed no wavelength
dependence between 500 and 700 nm. The signal monitored
at 900 nm (Figure 8b) was best fitted by the sum of a rise
having a lifetime of τ1 ) 518 ( 16 fs and a double-
exponential decay having lifetimes of τ2 ) 1.57 and τ3 )
14.7 ps. Careful scrutiny of the lifetimes of the time profiles
Figure 7. Evolution of the transient absorption spectra of CoNc(OBu)8 in
beyond 700 nm indicated that the first two lifetimes decreased
as the wavelength increased within each of the spectral
toluene (∼23 µM). (a) Transient absorption spectra in the visible region
following photoexcitation at 832 nm using a pump energy of 1 µJ/pulse.
envelopes separated by the isosbestic point, with ranges of
0.3-0.8 and 1.4-3.6 ps for τ1 and τ2, respectively.
Inset: enlarged area of the transient absorption evolution at times later in
the decay, showing isosbestic behavior. (b) Transient absorption spectra in
the near-IR region following excitation at 800 nm.
D. Theoretical Studies of the Ground and Excited States.
Molecular and Electronic Structure. Geometry optimi-
zations were performed with no symmetry constraints,
starting from nonsymmetric structures in which methyl
substituents residing on the same naphthoindole subunit had
the same orientation as in NiNc(OMe)8.10 Consistent with
the results of magnetic and electron paramagnetic resonance
(EPR) experiments showing that Co(II) and Cu(II) phtha-
locyanines contain one unpaired electron,51–54 the structures
(51) Assour, J. M.; Kahn, W. K. J. Am. Chem. Soc. 1965, 87, 207.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4281

Soldatova et al.
Figure 8. Kinetic behavior of transient absorption signals for CoNc(OBu)8 in toluene after excitation at 832 nm using probe wavelengths of (a) 650 and (b)
900 nm. Solid red lines are fits to the experimental data points.
Figure 9. Top view (left) and side view (right) of the DFT-optimized molecular structure of CoNc(OMe)8.
Table 2. Selected Bond Lengths (Å), Bond Angles (deg), and Metrical
of CoNc(OMe)8 and CuNc(OMe)8 were optimized by as-
Parameters Calculated for MNc(OMe)8 (M ) Co, Ni, Cu)
1
suming an S ) /2 spin state. In both cases, the system
evolved toward a symmetric structure with D2d symmetry
(see Figure 9). The calculated ground states of the cobalt
Co
Nia
Cu
M-Np
1.920
1.381
1.453
1.432
1.368
1.322
1.906
1.381
1.453
1.430
1.368
1.320
1.964
1.374
1.458
1.436
1.368
1.327
109.3
124.1
3.911
CR-Np
2
2
and copper complexes were A1 and B2, respectively. The
most relevant structural parameters calculated for CoN-
c(OMe)8 and CuNc(OMe)8 are gathered in Table 2 along
with those previously obtained for NiNc(OMe)8.10
CR-Cꢁ
Cꢁ-Cꢁ
Co-O
CR-Nb
CR-Np-CR
CR-Nb-CR
O· · ·O
CMe-O-Co-Cꢁ
(CR-Np-Np-CR)ad
107.9
122.8
3.880
63.1
11.2
107.6
122.2
3.865
63.0
11.3
Similar to the nickel analog, CoNc(OMe)8 and CuN-
c(OMe)8 have a saddled structure in which the methyl groups
are displaced from the plane of the pertinent naphthoindole
moiety. The dihedral angle between adjacent pyrrole ring
planes, (CR-Np-Np-CR)ad, which defines the degree of
saddling of the macrocycle, is virtually the same (∼11°) in
all three complexes. The torsion angle CMe-O-Co-Cb,
which provides a measure of the displacement of the CMe
atoms from the indole plane, is also substantially insensitive
to the nature of the coordinated metal. This is in line with
the tilting of the methyl groups out of the mean naphthalo-
63.0
11.3
b
a Data taken from ref 10. b Dihedral angle (deg) between adjacent pyrrole
ring planes.
cyanine plane being dictated by the necessity to minimize
the steric hindrance between the CMe-H and Cγ-H σ bonds.
According to the data in Table 2, the M-Np distance changes
significantly along the series. The trend in the calculated
M-Np distances, Co-Np (1.920 Å) > Ni-Np (1.906 Å) <
Cu-Np (1.964 Å), is consistent with that observed in the
corresponding phthalocyanines55–57 and, in the case of the
(52) Lever, A. B. P. J. Chem. Soc. 1965, 1821.
(53) Chen, I.; Abkowitz, M.; Sharp, H. J. Chem. Phys. 1969, 50, 2237.
(54) Guzy, C. M.; Raynor, J. B.; Symons, M. C. R. J. Chem. Soc. 1969,
2299.
(55) Williams, G. A.; Figgis, B. N.; Mason, R.; Mason, S. A.; Fielding,
P. E. J. Chem. Soc., Dalton Trans. 1980, 1688.
4282 Inorganic Chemistry, Vol. 47, No. 10, 2008

Photophysical BehaWior of CoNc(OBu)8 and CuNc(OBu)8
Figure 11. Spin density distribution in (a) CoNc(OMe)8 and (b)
CuNc(OMe)8.
CoPc.59,60 As expected for highly localized orbitals, the spin-
down counterpart of the 25a1v spin orbital, 27a1V, lies much
(∼2.5 eV) higher in energy, just below the substantially
unsplit 21b1 G-HOMO. The Co 3d contributions to the spin
orbitals indicate that among those shown in Figure 10, the
Figure 10. Energy-level scheme for CoNc(OMe)8 obtained from spin-
unrestricted DFT-BP/TZ2P calculations. In the numbering of the MOs, the
core electrons (1s for C, N, and O; 1s–2p for Co) are not included.
The Co 3d contributions to the spin orbitals are indicated in parentheses.
The 21b1 and 47e Gouterman MOs are indicated with red lines.
2
2
45e(dπ), 20b1(dxy), and 27b2(dx –y ) MOs also involve the
metal to a significant extent (it should be noted that we
identified the metal 3d orbital pointing to the pyrrolic
2
2
nitrogens as dx –y , although, according to the D2d choice of
axes, it is dxy). The doubly degenerate 45e MO results from
out-of-plane π interactions between the Co 3dπ orbitals and
occupied Np-based π orbitals of the macrocycle. Notably, a
sizable (29%) Co 3dπ contribution is also found in the spin-
down component of the higher-lying 46e orbital. The Co
3dπ also contributes to some extent (5%) to the nearly unsplit
47e G-LUMOs. The 45e, 46e, and 47e MOs all have
metal–ligand antibonding character. The 20b1 MO results
from an in-plane π antibonding interaction between the Co
3dxy orbital and a ligand orbital that is an Nb lone-pair orbital
with some in-plane Np pπ character. The unoccupied
last two members of the series, naphthalocyanines.58 If the
shortening of the M-Np bond length in going from Co to
Ni reflects the decrease of the ionic radius of the central
metal, the “anomalously” long M-Np bond length in
CuNc(OMe)8 has to be traced to single occupancy of the σ
antibonding molecular orbital (MO) formed from the Cu
3dx –y and Np lone-pair orbitals (discussed below). In
response to the lengthening of the M-Np distance, a
significant expansion of the macrocycle core occurs, as
evidenced by the increases in the CR-Nb bond distance and
the CR-Np-CR and CR-Nb-CR bond angles relative to those
in the nickel and cobalt analogs (Table 2).
2
2
2
2
27b2(dx –y ) MO is an antibonding Co-Np orbital composed
2
2
of almost equal amounts of Co 3dx –y and pyrrolic nitrogen
lone-pair orbitals.
An energy-level scheme showing the highest-occupied and
lowest-unoccupied Kohn–Sham spin orbitals of CoNc(OMe)8
is displayed in Figure 10. According to the DFT results, the
unpaired electron resides in the 25a1v spin orbital whose
According to the energy-level scheme in Figure 12, the
2
2
27b2(dx –y )v spin orbital becomes singly occupied in CuN-
c(OMe)8. Consistent with the fact that the SOMO (singly
occupied molecular orbital) is a nearly 50:50 mixture of Cu
2
spatial part is mainly a pure metal orbital, largely 3dz (76%)
with some 4s character (9%). Analysis of the electron density
distribution in the complex indicates that the spin density is
mostly located on the cobalt atom, with small pockets of
opposite spin found on the Np atoms [see Figure 11a for a
plot of the spin density distribution in CoNc(OMe)8]. This
is quite similar to what has been found in the case of
2
2
3dx –y and Np lone pair orbitals, the spin density in the
complex is almost equally distributed between the copper
atom and the pyrrolic nitrogens [see Figure 11b for a plot
of the spin density distribution in CuNc(OMe)8]. A similar
spin density distribution was found in CuPc.53,54 The metal
levels are significantly stabilized in going from the cobalt
(56) Schramm, C. J.; Scaringe, R. P.; Stojakovic, D. R.; Ibers, J. A.; Marks,
T. J. J. Am. Chem. Soc. 1980, 102, 6702.
(59) Figgis, B. N.; Kuckarski, E. S.; Reynolds, P. A. J. Am. Chem. Soc.
1989, 111, 1683.
(57) Brown, C. J. J. Chem. Soc. A 1968, 2488.
(58) Morishige, K.; Araki, K. J. Chem. Soc., Dalton Trans. 1996, 4303.
(60) Bialek, B.; Kim, I. G.; Lee, J. I. Thin Solid Films 2006, 513, 110.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4283

Soldatova et al.
band, with a maximum at 852 nm and an oscillator strength
of 0.690, was assigned to the 32E state, which was computed
to lie at 1.37 eV (905 nm), in satisfactory agreement with
experiment. This state is an almost pure G-21b1 f G-47e,
π f π* state. The compositions of the excited states given
in Table 3 indicate that the lower-lying 12E and 14E states
originate from the same transition. Following the Gouterman
notation,43 the 32E, 12E, and 14E states will hereafter be
2
2
4
designated S1(π, π*), T1(π, π*), and T1(π, π*), respec-
tively. According to the computed vertical excitation energies
2
4
in Table 3, the T1(π, π*) and T1(π, π*) states were the
best candidates for assignment to the weak absorptions
appearing at 1155 and 1445 nm, respectively, in the near-
IR spectrum of CuNc(OBu)8 (see Figure 2). There was good
agreement between the calculated excitation energies and the
experimental band maxima, although the 2T1/4T1 splitting was
somewhat underestimated theoretically. In addition to the
2T1(π, π*) and 4T1(π, π*) states, TDDFT calculations in the
gas phase also located three LMCT excited states below the
2S1(π, π*) state. Two of these, the nearly degenerate 22E
and 12A2 states, vertically lie immediately below 2S1(π, π*);
the third one, 12B1, with a computed energy of 0.38 eV, is
the lowest-energy excited state. The LMCT excited states
involve transitions from a π orbital of the macrocycle into
the 27b2V spin orbital, a σ antibonding MO formed from the
Cu 3dx –y and Np lone-pair orbitals. In regard to the LMCT
states, it should be pointed out that our classification of
LMCT states is based merely on the character of the MOs
involved in the transitions. In fact, no net charge transfer
from the ligand to the metal occurs; only a reorganization
of the electronic density takes place. This happens quite
frequently in so-called LMCT states of transition-metal
complexes, including metallophthalocyanines10,38 (for an
extensive discussion on this topic, see ref 61). Therefore,
the well-known failure of TDDFT to correctly predict the
excitation energies of charge-transfer states62–65 does not
occur here, and the TDDFT energies calculated for the
LMCT states can be trusted.
Figure 12. Energy-level scheme for CuNc(OMe)8 obtained from spin-
unrestricted DFT/BP/TZ2P calculations. In the numbering of the MOs, the
core electrons (1s for C, N, and O; 1s–2p for Cu) are not included.
The Cu 3d contributions to the spin orbitals are indicated in parentheses.
The 21b1 and 47e Gouterman MOs are indicated with red lines.
2
2
to the copper complex as a result of the increased nuclear
charge of the metal. As a consequence, in CuNc(OMe)8 there
are no metal states in the upper valence region apart from
2
2
the just-mentioned 27b2(dx –y ) MO.
Excited States Below the 2S1(π, π*) State. To gain insight
2
into the deactivation mechanism of the S1(π, π*) excited
state in CoNc(OBu)8 and CuNc(OBu)8, the nature and energy
of the excited states lying vertically between the 2S1(π, π*)
state and the ground state were explored theoretically. The
molecules studied here have doublet ground states, leading
to doublet and quartet excited states. One-electron transitions
from fully occupied orbitals to virtual orbitals result in one
set of quartet excited states and two sets of doublet states,
whereas one-electron transitions from or into the singly
occupied orbital result in doublet states.
As the photophysical experiments were performed in
toluene solution, and, in the case of the copper complex,
also in benzonitrile solution, the effects of the solvent on
the excitation energies were also explored.
Vertical excitation energies (Eva) calculated for the lowest
doublet and quartet states of CuNc(OMe)8 and CoNc(OMe)8
are gathered in Tables 3 and 4, respectively. Tables 3 and 4
also include adiabatic energies (Eadia) calculated for selected
excited states and the composition of the BP/ALDA solution
vectors in terms of the major one-electron MO transitions.
The TDDFT energies of the excited states in the gas phase
will be considered first, and then the solvent effects will be
examined.
The pattern of excited states derived from the vertical
excitation energies may be altered to some extent after
consideration of geometry-relaxation effects. Detailed theo-
retical investigations of the excited-state properties of several
nickel tetrapyrroles have clearly demonstrated that when
geometric relaxation of the excited states is taken into
account, the excitation energies of (d, d) and LMCT
transitions involving population of the strongly antibonding
2
2
σ Ni(dx –y )-Np(lone pairs) MO are shifted downward
significantly (0.13–0.54 eV).10,37,38 The π, π* excited states,
on the other hand, do not show significant relaxation,
although they undergo Jahn–Teller distortion. The adiabatic
excitation energies of the π, π* states were indeed predicted
(61) Rosa, A.; Ricciardi, G.; Gritsenko, O.; Baerends, E. J. Struct. Bonding
(Berlin) 2004, 112, 49.
(62) Casida, M. E.; Gutierrez, F.; Guan, J.; Gadea, F.-X.; Salahub, D.;
Daudey, J.-P. J. Chem. Phys. 2000, 113, 7062.
(63) Dreuw, A.; Weisman, J. L.; Head-Gordon, M. J. Chem. Phys. 2003,
119, 2943.
CuNc(OBu)8. On the basis of the calculated vertical ex-
citation energies and oscillator strengths, the intense Q(0, 0)
(64) Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007.
(65) Gritsenko, O.; Baerends, E. J. J. Chem. Phys. 2004, 112, 655.
4284 Inorganic Chemistry, Vol. 47, No. 10, 2008

Photophysical BehaWior of CoNc(OBu)8 and CuNc(OBu)8
Table 3. Excitation Energies of the Lowest Excited States of CuNc(OMe)8
gas-phase
toluene/benzonitrile
Eva (eV)b
exptl
E (nm/eV)b
statea
32E
compositiona (%)
type
Eva (eV)b
Eadia (eV)
Eadia (eV)
21b1v f 47ev (45)
21b1V f 47eV (49)
46eV f 27b2V (100)
19a2V f 27b2V (100)
21b1v f 47ev (52)
21b1V f 47eV (48)
21b1V f 47ev (100)
21b1V f 27b2V (100)
π, π*
1.37 (0.662)
1.37/1.38 (0.698/0.701)
851/1.46 (0.690)c
Q(0, 0)
22E
LMCT
LMCT
π, π*
1.33 (0.0001)
1.27
0.91 (0.0003)
1.43/1.50 (0.0001/0.1740)
1.37/1.46
0.91/0.92 (0.0003)
12A2
12E
1.10d
0.25d
1.20e/1.29e
0.33e/0.40e
1155/1.07 (0.0008)f
1440/0.86 (0.0005)f
14E
π, π*
LMCT
0.89
0.38
0.90/0.90
0.48/0.57
12B1
a Numbering and composition of the excited states refer to TDDFT calculations in the gas phase. b Oscillator strengths in parentheses. c In toluene solution,
from this work. d Geometry optimization of the excited-state structure was performed under the constraint of D2d symmetry. e Computed at the geometry
optimized in the gas phase (D2d symmetry). f In CH2Cl2 solution, from this work.
Table 4. Excitation Energies of the Lowest Excited States of CoNc(OMe)8
gas-phase
Eva (eV)b
toluene
exptl
E (nm/eV)b
statea
42E
compositiona (%)
type
π, π*
Eadia (eV)
Eva (eV)b
Eadia (eV)
21b1v f 47ev (45)
21b1V f 47eV (49)
21b1v f 27b2v (100)
21b1V f 27b2v (100)
20b1V f 27a1V (100)
45eV f 27a1V (99)
21b1v f 47ev (52)
21b1V f 47eV (47)
21b1V f 47ev (100)
19a2V f 27a1V (100)
46eV f 27a1V (99)
1.39 (0.662)
1.39 (0.660)
832/1.49 (0.540)c
Q(0, 0)
22A2
14A2
12B1
32E
LMCT
LMCT
1.27
1.26
1.20
1.02 (0.00003)
0.95 (0.0006)
1.31
1.30
1.24
2
dxy, dz
2
LMCT/(dπ, dz )
π, π*
1.15 (0.0002)
0.96 (0.0006)
22E
14E
π, π*
LMCT
LMCT/(dπ, dz )
0.93
0.94
0.89
1332/0.93 (0.0006)d
12A2
12E
0.74
0.63 (0.00002)
0.61e (12B1)
0.74 (0.00001)
0.72e,f(12B1)
2
a Numbering and composition of the states refer to TDDFT calculations in the gas phase. b Oscillator strengths in parentheses. c In toluene solution, from
this work. d In CH2Cl2 solution, from this work. e The energy value refers to the Jahn–Teller-distorted C2V geometry. f Computed at the geometry optimized
in the gas phase (C2V symmetry).
to be less than 0.1 eV lower than the corresponding vertical
absorption energies.10,38 In this work, geometry-relaxation
effects on the excited states were analyzed only for the lowest
2LMCT state, 12B1, and one of the two 2LMCT states (12A2)
lying vertically between the 2S1(π, π*) and 2T1(π, π*) states
(the behavior of the 22E state was expected to be similar).
The relative positions of the 12A2 and 22E LMCT states with
respect to the 2,4T1 set and of the 12B1 state with respect to
the ground state are expected to be crucial in interpreting
expansion of the macrocycle core. These geometrical changes
2
2
are clearly related to occupation of the 27b2(dx –y )V spin orbital.
The TDDFT results in toluene (ꢀ ) 2.38) and benzonitrile
(ꢀ ) 26.00) reported in Table 3 show that the vertical
absorption energies of the excited states with π, π* character
were scarcely (if at all) affected by solvation. However, the
vertical absorption energies of the LMCT states were shifted
upward by ∼0.1 eV in toluene and ∼0.2 eV in benzonitrile.
As a result, the higher-energy 2LMCT states did not lie below
2
the pathway for deactivation of the S1(π, π*) state in
CuNc(OBu)8.
2
the S1(π, π*) state in these solvents. In turn, the lowest
LMCT state, 12B1, moved closer to the 4T1 state, particularly
in benzonitrile. Interestingly, the oscillator strength of the
22E LMCT state in benzonitrile was significantly larger than
that in either toluene or the gas phase (see Table 3), which
can be explained by the admixture of the more-intense 27b2v
f 47ev transition into the 46eV f 27b2V transition.
The Eadia value computed for the 12A2 excited state (see
Table 3) suggests that the minimum of the relaxed energy
surface of this state (and presumably that of 22E also) is well
above the minima of the relaxed energy surfaces of the 2T1(π,
4
π*) and T1(π, π*) states. Hence, the 12A2 and 22E LMCT
states are expected to play a role only at the early stage of
the deactivation mechanism. As for the lowest LMCT state,
12B1, the minimum of its relaxed energy surface is predicted
to lie only 0.25 eV above the minimum of the ground-state
surface. The significant decreases (0.17 and 0.13 eV,
respectively) in the adiabatic energies of the 12A2 and 12B1
LMCT states with respect to the corresponding vertical
absorption energies indicate that the relaxed geometries of
these states show significant changes compared to the
ground-state geometry. As can be seen in Table S1 in the
Supporting Information, which contains optimized geo-
metrical parameters of the 12A2 and 12B1 excited states (and,
for the sake of comparison, those of the ground state), the
relaxed geometries of these states are characterized by a
sizable (∼0.04 Å) lengthening of the Cu-Np distances and
In regard to the solvation model adopted in the calculations
of the excited states, it should be stressed here that since the
COSMO surface charges are not explicitly considered in the
TDDFT procedure other than through their influence on
the ground-state density (see Quantum Chemical Calculations
in the Experimental Section), changes in vertical excitation
energies upon solvation only reflect changes in the energies
of the ground-state molecular orbitals. In toluene and, to a
greater extent, in benzonitrile, the occupied and virtual ligand
orbitals undergo a generalized stabilization, whereas the spin-
2
2
up and spin-down components of the 27b2(dx –y ) MO do not
experience significant changes in energy. This explains why
the excited states with π, π* character are affected very little
by solvation and the LMCT transitions undergo an upshift.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4285

Soldatova et al.
The higher the polarity of the solvent, the more pronounced
the upshift is.
When both solvation and geometry-relaxation effects were
taken into account, the minimum of the relaxed energy
surface of the 12A2 state was moved back between those of
the 2S1(π, π*) and 2T1(π, π*) states (see Eadia in Table 3). It
is not unreasonable to assume that this holds true also for
the 22E LMCT state. As for the lowest LMCT state, 12B1,
the minimum of its relaxed energy surface was predicted to
lie at 0.33 eV in toluene and 0.40 eV in benzonitrile,
indicating that the energy gap between this state and the
higher-energy 4T1 state decreases when the dielectric constant
of the solvent is increased.
CoNc(OBu)8. The vertical absorption energies and oscil-
lator strengths of the excited states of the cobalt complex
given in Table 4 clearly indicate that the 42E state, which
was computed to lie at 1.39 eV (892 nm) in both the gas
phase and toluene solution, is responsible for the intense Q(0,
0) band with a maximum at 832 nm and an oscillator strength
of 0.540. The calculations also account for the small (∼20
nm) blue shift of this band observed in moving from
Figure 13. Spin density distribution of CoNc(OMe)8 in the 12B1 excited
state.
relaxed geometry. The 12E state is doubly degenerate and is
expected to undergo Jahn–Teller distortion along the b1 or
b2 active mode into a D2 or C2V structure, respectively.
Indeed, geometry optimization of 12E indicated that the D2d
symmetry was no longer stable for this state and was lowered
to C2V. The resulting 12B1 electronic state failed to show
significant geometrical relaxation, as can be inferred from
the geometrical parameters gathered in Table S2 in the
Supporting Information. In line with the minor conforma-
tional relaxation, the adiabatic energy of the 12B1 state was
only 0.01 eV lower than the vertical absorption energy (see
Table 4). Analysis of the electron density distribution of
CoNc(OMe)8 in its 12B1 excited state indicated that the spin
density is mainly localized on the cobalt atom, having Co
3dxz as the contributing orbital, with very small districts of
opposite spin on the Np atoms (see Figure 13 for a plot of
the spin density distribution). Accordingly, the Mulliken
charge on the cobalt atom in the 12B1 state did not change
significantly with respect to the ground-state value (+0.63
vs +0.71).
2
CuNc(OBu)8 to CoNc(OBu)8. The T1(π, π*) (22E) and
4T1(π, π*) (14E) excited states were computed to have gas-
phase energies of 0.95 and 0.93 eV, respectively, and nearly
identical Eva values were obtained in toluene (Table 4).
2
4
The T1(π, π*) and T1(π, π*) excited states, as well as
the 2S1(π, π*) state with which they share a common
electronic origin, were computed to have slightly higher
energies in the cobalt complex than in the copper analog.
4
The vertical absorption energy computed for T1(π, π*)
suggested that this state be assigned to the weak feature (f
) 0.0006) appearing at 1332 nm in the near-IR spectrum of
CoNc(OBu)8. The observed blue shift of this band in going
from the copper to the cobalt complex (1332 vs 1440 nm)
further supports this assignment.
TDDFT results indicate that four excited states lie verti-
2
cally between the S1(π, π*) state and the 2,4T1(π, π*) set.
Therefore, if populated, the 12E (12B1) excited state is
expected to show the derivative-shaped transient absorption
difference spectrum characteristic of a (d, d) state. The
excited-state pattern as derived from gas-phase TDDFT
calculations was not modified when solvent effects were
taken into account.
E. Deactivation Mechanism of the Excited States. To
elucidate the mechanism of the excited-state deactivation and
gain an insight into the effect of the metal on the relaxation
processes, the results of the transient absorption experiments
on the first-row transition-metal naphthalocyanines consid-
ered here were assimilated with those of TDDFT studies.
The combination of theoretical and experimental results can
provide a comprehensive picture of the photophysical
properties of the compounds and an understanding of the
fundamental processes that take place after photoexcitation.
This knowledge is essential for pursuing the idea of using
these compounds in photothermal therapy.
These are the 22A2, 14A2, 12B1, and 32E states. According to
the composition of the BP/ALDA solution vectors in Table
4, the nearly degenerate 22A2 and 14A2 states have LMCT
character involving transitions from the G-HOMO into the
Co-Np σ antibonding 27b2v spin orbital. The 12B1 state has
pure (d, d) character, while the 32E state has mixed LMCT/
2
(dπ, dz ) character, since it involves a transition out of 45eV,
which is largely a π orbital of the macrocycle with a 35%
contribution from Co 3dπ. The TDDFT results in toluene
show that none of these states was significantly affected by
solvation. Further inspection of the vertical absorption
energies gathered in Table 4 indicates that there are two
close-lying excited states, 12A2 and 12E, located vertically
below the 2,4T1(π, π*) set. The former is a pure 19a2V f
27a1V LMCT state, while the latter (similar to the higher-
2
2
energy 3 E state) has mixed LMCT/(dπ, dz ) character. Since
the results of the ultrafast experiments (Figure 7) clearly
demonstrated participation of a (d, d) state in the ground-
state repopulation process and the 12E state is the most
plausible candidate, we decided to ascertain whether the (d,
d) character of this state became more pronounced in its
CuNc(OBu)8. The earliest absorption spectral feature
detected in CuNc(OBu)8 after photoexcitation with 800 nm
light was a broad positive band between 510 and 740 nm
with negative bands at wavelengths where the ground-state
4286 Inorganic Chemistry, Vol. 47, No. 10, 2008

Photophysical BehaWior of CoNc(OBu)8 and CuNc(OBu)8
species absorbs, i.e., at 480, 755, and 850 nm (Figure 4a,b).
By analogy with the Ni(II) variant,10 this first observed
transient (FOT) was assigned to an excited state localized
on the π system, most likely the 2S1(π, π*) state (see below).
During the first few hundreds of femtoseconds of the
experiment, the spectral shape of this transient underwent
only minor changes characterized by the small rise of the
positive absorption (Figure 4a inset). The lifetime of this
rise was estimated to be ∼200 fs (Figure 6). Given the
similarity of the S1(π, π*) and T1(π, π*) absorption spectra
previously observed for metallophthalo- and metallonaph-
thalocyanines,10,24,25 we conclude that this ∼200 fs process
is internal conversion within the doublet manifold (2S1 f
2T1). Support for this assignment comes from the observation
of weak fluorescence from the Q state observed during the
static emission experiment (Figure 3). The measured fluo-
rescence quantum efficiency of ∼3 × 10-4 is consistent with
a lifetime of 200 fs for the emitting state. The calculations
Figure 14. Normalized kinetic profiles of the transient absorption signals
at 850 nm for CuNc(OBu)8 in two different solvents obtained after
photoexcitation at 800 nm.
process supports the contention that the rate-determining step
in the deactivation involves, at least in part, an LMCT state.
In the higher-dielectric medium, the LMCT state, which
remains spectrophotometrically undetectable, causes the
ground-state repopulation process to be more efficient as a
result of the energy shift relative to the ground state. Indeed,
when the energy of the 12B1 LMCT state was recalculated
in toluene and benzonitrile solution (Table 3), sizable
destabilization of this state in the more polar solvent
(benzonitrile) was observed, with the LMCT state located
2
done here (Table 3) indicate that at least one LMCT state
(the 12A2) lies adiabatically between the 2S1(π, π*) and 2T1(π,
π*) states. This intermediate state is likely to facilitate the
2
2S1 f T1 internal conversion.
2
The decay of the T1 state to the ground state showed
excellent isosbestic behavior and followed a single-expo-
nential kinetic law characterized by a lifetime of ∼1 ns. The
2
2
isosbestic behavior indicates that the T1 f S0 transition
occurs without the intervention of states having significant
lifetimes compared with 1 ns. Table 3 shows that a 4T1 state
2
0.1 eV closer to the T1 state compared to the situation in
toluene; this result supports the experimental observation of
faster ground-state repopulation in the more polar solvent.
CoNc(OBu)8. The evolution of the transient absorption
of CoNc(OBu)8 in toluene solution after photoexcitation at
845 nm (Figure 7) revealed two phases in the process of
relaxation to the ground state. The first phase was character-
ized by a fast decay (having a lifetime of ∼1 ps) of the
positive absorption signal stretching from 500 to 720 nm
accompanied by a complex evolution of the alternating
negative and positive absorption bands in the 740–760 and
820–920 nm regions. This complex evolution included blue-
shifting and narrowing of the derivative-shaped signal
accompanied by wavelength-dependent kinetics. The second
phase involved a decay of the derivative-shaped band to a
zero baseline with isosbestic behavior and a lifetime of 15
ps (Figure 7a inset). The observed feature in the difference
spectrum having alternating negative and positive lobes has
been well established to arise from the formation of a metal-
excited (d, d) state.37,38,66–69 Thus, it can be concluded that
the second kinetic phase represents repopulation of the
ground state from a metal (d, d) state. However, the measured
lifetime of 15 ps is much shorter than the typical lifetimes
of 220–500 ps exhibited by the lowest (d, d) states in
common Ni porphyrins37,70,71 and phthalocyanines10,38 but
comparable to that measured for Co(II) porphyrins (∼12
2
lies very close in energy to the T1 state, and below that is
an LMCT state (12B1). Either or both of these could be
involved in ground-state repopulation, but the transient
absorption data do not provide insight on this question. In
the static luminescence experiment (Figure 3), the CuN-
c(OBu)8 complex showed a weak shoulder at ∼1172 nm in
the red tail of the Q-state fluorescence and a distinct peak at
1460 nm; these features were assigned (see above) to
2
4
radiative transitions from the T1 (tentative) and T1 states,
4
respectively. This suggests that the T1 state plays a role in
the overall deactivation of the 2T1 state. However, the extent
of this role is probably quite small, as the emission signals
were extremely weak. On balance, the tentative conclusion
2
2
4
regarding deactivation of the T1 state is that the T1 f T1
2
2
and T1 f LMCT processes are competing deactivation
pathways, with the latter, a spin-allowed process, being
strongly favored. In both cases, the intermediate states would
have to have short lifetimes (<100 ps) in order to allow the
observed isosbestic behavior.
If an LMCT state were involved, its energy would be
sensitive to a change of solvent polarity. To investigate this
possibility, the pump–probe experiment was repeated in
benzonitrile solution. The results showed that the decay of
2
the T1 state was enhanced and the isosbestic behavior was
retained. Transient kinetic profiles taken at 850 nm and
normalized to unit ∆A are shown for toluene and benzonitrile
solutions of CuNc(OBu)8 in Figure 14. In benzonitrile,
repopulation of the ground state is clearly faster (having a
lifetime of ∼800 ps). This effect of a solvent with a high
dielectric constant on the rate of the ground-state repopulation
(66) Rodriguez, J.; Holten, D. J. Chem. Phys. 1989, 91, 3525.
(67) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111,
6500.
(68) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Chem. Phys. 1991, 94, 6020.
(69) Kim, D.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 584.
(70) Kim, D.; Holten, D. Chem. Phys. Lett. 1983, 98, 584.
(71) Kim, D.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4287

Soldatova et al.
ps).72 The TDDFT results (Table 4) suggest that the
2
2
experimentally observed (d, d) state is the 1 E(dπ, dz ) state,
and its short lifetime (∼15 ps) is consistent with this state
having quite a low energy (0.72 eV in toluene) and the same
multiplicity as the ground state.
In regard to the FOT, the broad positive absorption around
600 nm is reminiscent of the spectra of (π, π*) excited states
of other naphthalocyanine complexes.10,16,73 Since no detect-
able Q-state fluorescence was observed during the static
luminescence measurements (in contrast to what was ob-
served for the Cu complex), it is very likely that the Q-state
lifetime is extremely short and that this state generates the
2T1 state within the instrument response time (∼200 fs).
Moreover, the TDDFT computations (Table 4) showed the
existence of several excited states between the Q state and
Figure 15. Schematic diagrams showing the proposed excited-state
relaxation pathways for (left) CoNc(OBu)8 and (right) CuNc(OBu)8 in
toluene.
2
the T1 state; these are likely to assist the efficient internal
2
conversion of the Q state to the T1(π, π*) state. The
2
participation of the T1 state in the deactivation pathways
2
2
hot (dπ, dz ) state, decays to the ground-state surface while
could be further supported by the weak emission observed
for the cobalt complex at ∼1092 nm.
the metal-centered state undergoes vibrational relaxation. The
2LMCT state, theoretically predicted to lie below the set of
The significant positive absorption band detected to the
red of the Q-band bleaching of the FOT is characteristic of
a hot (d, d) state, as observed in most nickel porphyrins and
phthalocyanines37,66,68,74–76 but not observed here in the case
of the copper variant, which showed no (d, d) state formation.
Thus, the first observed transient is probably a composite of
the absorptions of the 2T1(π, π*) and metal-centered 12E(dπ,
2
triplets, enhances internal conversion from the T1 state to
the ground state.
In summary, after the Q state is generated by photoexci-
tation, it deactivates within the instrument response time
through two channels. One channel leads to the formation
2
of the T1 state localized on the π system, while the other
2
2
leads to the vibrationally hot (dπ, dz ) state. Thus, the FOT
2
dz ) excited states. The question is whether the π-localized
is a superposition of the absorption spectra of the 2T1(π, π*)
and hot (dπ, dz ) states. The T1(π, π*) state decays back to
the ground state with a lifetime of 1 ps while the (d, d)
state, the broad positive absorption at 650 nm with a lifetime
2
2
2
of 1 ps, decays directly to the ground state or forms the 2(dπ,
2
2
dz ) state.
state experiences a vibrational cooling process. After the
cooling process is complete, the 2(d, d) state repopulates the
ground state with a lifetime of 15 ps.
To summarize the above conclusions, diagrams illustrating
the pathways for deactivation of the excited Q states of the
Co(II) and Cu(II) octabutoxynaphthalocyanine complexes in
toluene are shown in Figure 15.
In the 550–710 nm region, where the ground state of
CoNc(OBu)8 absorbs negligibly or not at all, the evolution
of the transient absorption signal represents only the decay
of the (π, π*) excited state(s). However, in the 740–840 nm
region, where the ground state has a significant extinction
coefficient, the kinetic traces represent a superposition of the
temporal progressions of both the positive absorption and
the ground-state recovery. Thus, the evolution of the transient
absorption signal in this region is a sum of these two
processes weighted by contributions from the extinction
coefficients of the ground- and excited-state species. The
reduction in the intensity of the negative absorption bands
observed in Figure 7 during the first 3-4 ps is too large to
Concluding Remarks
Transient absorption experiments have shown that chang-
ing the central metal M from Co through Ni to Cu results in
striking differences in the photophysical behavior of the
MNc(OBu)8 complexes. Variations in the lifetimes for
repopulation of the ground state from tens of picoseconds
to nanoseconds were observed across the MNc(OBu)8 (M
) Co, Ni, Cu) series. These differences can be rationalized
in terms of significant changes induced by the central metal
in the nature and energies of the excited states involved in
the decay route of the primarily photogenerated Q state. It
is instructive in this regard to compare the photophysical
behavior of NiNc(OBu)8 and CuNc(OBu)8. For both of these
complexes, transient absorption experiments and TDDFT
calculations suggest that repopulation of the ground state
proceeds from triplet states localized on the π system with
2
2
be explained solely by an increase in the (dπ, dz ) extinction
coefficient at these wavelengths during the vibrational
cooling process. The large decrease in bleaching intensity
is most likely to be the result of repopulation of the ground
state. Thus, the 2T1 state, which is populated from the initially
produced Q state in competition with the formation of the
(72) Loppnow, G. R.; Melamed, D.; Leheny, A. R.; Hamilton, A. D.; Spiro,
T. G. J. Phys. Chem. 1993, 97, 8969.
(73) Firey, P. A.; Rodgers, M. A. J. Photochem. Photobiol. 1987, 45, 535.
(74) Eom, H. S.; Jeoung, S. C.; Kim, D.; Ha, J. H.; Kim, Y. R. J. Phys.
Chem. A 1997, 101, 3661.
(75) Drain, C. M.; Kirmaier, C.; Medforth, C. J.; Nurco, D. J.; Smith, K. M.;
Holten, D. J. Phys. Chem. 1996, 100, 11984.
2
2
participation of the LMCT(π, dx –y ) state. However, in the
Cu complex, this process takes twice as long as in the Ni
complex. The vertical absorption energies calculated for the
(76) Drain, C. M.; Gentemann, S.; Roberts, J. A.; Nelson, N. Y.; Medforth,
C. J.; Jia, S.; Simpson, M. C.; Smith, K. M.; Fajer, J.; Shelnutt, J. A.;
Holten, D. J. Am. Chem. Soc. 1998, 120, 3781.
4288 Inorganic Chemistry, Vol. 47, No. 10, 2008

Photophysical BehaWior of CoNc(OBu)8 and CuNc(OBu)8
T1(π, π*) state in the nickel complex and the 2,4T1(π, π*)
pair of states in the copper complex do not differ signifi-
cantly; however, the LMCT state of the Cu complex is much
lower in energy than that of the Ni complex (0.38 vs 0.84
eV), with the result that in the Cu complex, the rate-
implemented in ADF would be adequate. Unfortunately,
ZORA spin–orbit-coupled TDDFT calculations are currently
restricted to closed-shell systems.
In spite of the differences observed in their photophysical
behavior, CoNc(OBu)8 and CuNc(OBu)8 (as well as the
previously investigated Ni variant) fulfill the fundamental
requirements for consideration as photothermal sensitizers
for in vivo use: they have intense, far-red optical absorptions,
and their excitation energies are rapidly dissipated through
radiationless channels. The results described in this work
present necessary background and basic knowledge in
support of the prospective use of these compounds in
photothermal therapeutic applications.
2
determining process is the intrinsic decay of the T1(π, π*)
state to the lower-lying LMCT state. In the Ni variant, the
LMCT state is in equilibrium with the nearly degenerate
T1(π, π*) state, resulting in the observed facile (∼500 ps)
repopulation of the ground state.
Nevertheless, a thorough understanding of the variations
in the ground-state repopulation rates as a function of the
coordinated metal remains elusive. The underlying difficulty
in this quest is that theory provides thermodynamic data (i.e.,
the energies of states that possibly contribute to the decay
routes), which is not necessarily relevant to an examination
of rate processes. What is really required is information about
the topography of the excited-state surfaces, such as the
locations of intersections and the heights of energy barriers.
However, for large molecules such as those considered here,
such information is beyond the realm of the extant method-
ologies. As a matter of fact, location of intersections has
been done for small molecules using TDDFT methods,
although the applicability of TDDFT using modern func-
tionals when conical intersections are involved is currently
not completely clear.77
Another question concerning the photophysics of the
copper and cobalt complexes that remains unanswered is
whether, and to what extent, the 4T1 state is involved in the
deactivation mechanism. Knowledge about the efficiency of
the 2T1 f 4T1 intersystem crossing process would represent
a step forward in this direction. This could be achieved
through explicit calculations of the spin–orbit coupling
between these states, for which the relativistic spin–orbit
zero-order regular approximation (ZORA) formalism78–80
Acknowledgment. The photophysical studies conducted
at the Ohio Laboratory for Kinetic Spectrometry (BGSU)
were supported in part by NIH Grant CA 91027 and NIH
Grant CA 48735 (M.E.K.) as well as by an instrumentation
grant from the Hayes Investment Foundation (Ohio Board
of Regents). A.V.S. thanks the McMaster Foundation at
BGSU for a predoctoral fellowship. A.R. and G.R. thank
the Italian MIUR (Ministero dell′ Istruzione, dell′ Università
e della Ricerca) and the Università della Basilicata for Grant
2003038084_002. Sincere thanks are expressed to Dr.
Malcolm Chisholm of The Ohio State University for
graciously making time available on his NIR spectrometer
and to Dr. Eugene Danilov for technical assistance with the
ultrafast instrumentation.
Supporting Information Available: Optimized geometrical
parameters for the 12A2 and 12B1 excited states of CuNc(OMe)8
(Table S1) and the 12B1 excited state of CoNc(OMe)8 (Table S2).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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